1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier or a printer, for forming an image on a recording medium using a recording technique, such as an electrophotographic recording method or an electrostatic recording method. In particular, the present invention relates to an image forming apparatus having a rotary unit for holding multiple developing devices.
2. Description of the Related Art
A developing device, which uses toner to develop an electrostatic latent image formed on a photosensitive member, is mounted in an image forming apparatus, such as an electrophotographic copier or printer. Generally, provided for the developing device are a developing roller and a blade for controlling the thickness of a toner layer.
Especially when the image forming apparatus is one for which the developing roller is a flexible roller that is contacted by the blade, if the developing device is not used for an extended period of time, a blade print is formed, and retained, on the developing roller and produces images having uneven densities.
To avoid the deterioration of the developing characteristic due to the blade print, or due to another factor, countermeasures are proposed in JP-A-8-227211, JP-A-2000-227710, JP-A-8-62923, JP-A-11-73012 and JP-B-7-117783. According to these methods, preparative rotation of the developing roller is performed immediately before the developing process is begun, or rotation of the developing roller is started after a predetermined time has elapsed following the completion of the image forming.
When the preparative rotation of a developing roller is performed immediately before the developing, the period before the developing process is started is too short, and complete removal of a blade print can not be expected. Whereas if the performance of the preparative rotation takes too long, the period required for the printing will be extended. Further, if rotation of the developing roller occurs after a predetermined time has elapsed following termination of the image forming, power consumption in the standby state, wherein image forming is not performed, will be increased.